We caught up with Harley last week in Montauk and immediately had to ask him: what else can you debunk? But the number one thing he wants to clear up isn't about food. It's about running.

"You don't have to run to burn fat," he told POPSUGAR. "I have clients who come in and they say, 'My knee really hurts; I'm so scared I can't run.' We say, 'Can you walk?' but they say, 'Yeah, but I don't burn fat walking.' Well, that's not a thing!"

You actually burn more fat when you walk than when you run, because of the way our body uses energy stores. "Believe it or not, a greater percentage of your calories come from fat when you walk rather than when you run," he said. "The faster you run, the greater percentage of your calories come from carbohydrates."

"I don't care if you Salsa, I don't care if you Lambada, if you walk, if you go to get coffee a lot throughout the day . . . whatever it is, it's about moving your body."

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And you don't just benefit in the short-term from walking's fat-burning capabilities. Walking can also keep you healthier for longer. "It's not only the intensity of cardio, but the duration, the wear and tear on your body," said Harley. "That all needs to be taken into account. You might feel really great running, but if you run too much, too often, at some point, your body's going to give in."

He also noted that intense cardio can make you intensely hungry. "There's something about the relationship about your intensity of cardio and your appetite . . . . You've heard the saying work up an appetite? With Spin classes, they almost always open up a juice shop across the way."

It's no secret that his mantra is "Strong in the gym, lean in life." It's one of the biggest reasons he feels so passionately about the Fitbit model; it's about movement more than exercise. "With my Fitbit, it liberated me and my clients from this idea of 'How many minutes do I have to do cardio for?' — I don't care. 'How hard to I have to do cardio?' — I don't care. It's all about steps."